This invention relates generally to reference apparatus for navigable vehicles such as aircraft, and more particularly, to rate/acceleration sensors used in an attitude and heading reference system. Advancement in the art of precision flight control and guidance apparatus for aircraft, missiles, and space vehicles depends in part on progress in sensor technology. Present computer technology allows sophisticated and complex signal processing at reasonable cost, but the information processed is frequently derived from sensors having a cost which is a disproportionate part of the system cost.
A reference system having inertial instruments rigidly fixed along a vehicle-based orientation reference wherein the instruments are subjected to vehicle rotations and the instrument outputs are stabilized computationally instead of mechanically is termed a gimballess or strapped-down system. Such systems generally include computing means receiving navaid data such as magnetic and radio heading; air data such as barometric pressure, density, and air speed; along with output signals of the inertial instruments for generating signals representative of vehicle position and orientation relative to a system of coordinate axes, usually earth oriented. The presence of high angular rates associated with strapped-down systems adversely affects performance and mechanization requirements. Consequently, such reference systems have been used extensively in missiles, space, and military vehicles, but their use in commercial aircraft has been less extensive because of economic constraints associated with the manufacture of precision mechanical assemblies, i.e., gyroscopes and other precision sensors. Strapped-down inertial reference systems become practical for commercial aircraft from the standpoint of cost of ownership, weight, reliability, and maintainability with the advent of small, lightweight, highly accurate and relatively low-cost rate sensors and accelerometers. Angular rate sensing apparatus utilizing rotating piezoelectric generators are known; see for example U.S. Pat. Nos. 2,716,893 and 4,197,737. Such devices generally comprise piezoelectric generator elements mounted to a rotatable drive shaft and oriented for generating signals responsive to particular bending forces sensed by the instrument; the processing of signals derived from such instrumentation involves the measurement, amplification and transmission of very low level DC and low frequency signals. Prior art devices have exhibited signal degradations which make the devices unsuitable for some applications. For example, the signals being processed may contain undesirable carrier harmonics, DC bias, and other noise components, such as those caused by signal phase shifts and mechanical misalignments in the system, which undesirable components must be rejected to prevent degradation of the low-level signals of primary interest.
In view of the problems of present state-of-the-art sensors, described above, it is a general objective of the present invention to develop an improved low-cost sensor for generating signals representative of vehicle accelerations and angular rates.
A more specific object of the invention is to provide an improved multi-function sensor for measuring angular rate about two axes and linear acceleration along two axes, and having relatively simple mechanical construction, low bias drift, and high sensitivity.
It is another object of the invention to provide an improved linear acceleration sensor wherein undesired signal components such as those caused by vibrations at the second harmonic of the spin frequency are removed from the output signal.